The present invention relates to a vent for a rechargeable battery and, more particularly this invention relates to a vent for a zinc alkaline battery.
Electrolytic reactions, especially those reactions occurring in aqueous electrolyte can produce gas during charging, discharging or over charging operations. Many batteries are cathode or anode limited to prevent generation of gas. Uncontrolled generation of gas can disrupt a cell compartment and/or lead to catastrophic destruction of the cell with attendant hazard of injuring personnel with a spray of debris and of acid or basic electrolyte.
Gas buildup in certain electrolytic reactions is somewhat unavoidable such as in alkaline batteries due to corrosive influence of the alkaline electrolyte on charged active metal species in the anode according to the following reaction:
M+H2Oxe2x86x92MOH+1/2H2
Where xe2x80x9cMxe2x80x9d represents a metal species such as zinc, cadmium, iron, nickel and the like.
A particularly severe problem arises when a seemingly small gas buildup leads to increased battery impedance, which in turn, depending on the charging scheme used, can lead to further gas production. Thus, a positive feedback loop is established.
Vents have been utilized to provide controlled release of gas generated within a battery case. Ideally, a battery should not vent, for material lost through a vent represents loss of reactant and departure from the optimum conditions originally designed for the battery. Since gas formation in a rechargeable alkaline battery, particularly at the end of the charging cycle, is to some extent unavoidable, it is necessary that a venting mechanism be provided.
Deformable elements in batteries have been utilized to exhaust gas upon build-up from a battery. Kilb in U.S. Pat. No. 5,789,096 describes a valve with a deformable element comprising a thermoplastic elastomer such as a modified polyamide, a styrene-ethylenebutylenestyrene copolymer, and/or polypropylene. This valve is designed to open during increased pressure and to close under normal pressure. U.S. Pat. No. 4,020,241 by Heinz, Jr. shows a resilient elastomeric sponge gasket disposed between the battery cell and the container of the cell along the cover-gasket interface that vents in the 5 to 75 psi region.
Valves have also been prominently utilized to gradually vent gas from batteries. In U.S. Pat. No. 5,554,455 Inoue et al. describe a valve which projects from a hole in the battery, comprising a moving element and a coil spring. An electrical valve is described in U.S. Pat. No. 5,741,606 by Mayer et al. Internal pressure within the battery opens up an electrical contact which places the battery in open circuit. A backup pressure relief system consists of a pressure rupturable region in the flap portion of the system. Wolff in U.S. Pat. No. 4,822,377 depicts a compressible deformable vent plug operable in the range of 200 to 500 psi. Winsel in U.S. Pat. No. 4,180,624 describes two valves in the battery housing, one which responds to an overpressure and the other one to underpressure. Heinz, Jr. in U.S. Pat. No. 4,052,533 discloses a cup-shaped flapper vent valve that deflects upwardly with pressure.
Holes in battery cases are also used as vent elements. U.S. Pat. No. 5,702,840 describes two holes in a depressed region of a lithium battery, said region being filled with electrolyte. Ng et al. in U.S. Pat. No. 4,965,144 describe a hole which transmits pressure to an O-ring. In the event of excess pressure within the container, the O-ring is displaced by the excess pressure to vent the container and the O-ring returns to its original sealing position when the excess pressure is relieved. Messing in U.S. Pat. No. 4,064,323 teaches the use of a metal foil which is sandwiched between two gas pervious plastic layers. The metal foil fractures in the case of buildup which allows gas transmission.
In the present invention, the case is not modified. Rather, the present invention utilizes a multistage venting system. First, the case is made of a particularly high gas transport material. A one-way valve, in the form of a metallized plastic envelope tightly wrapped around the battery, expands upon pressure accumulation. Gas is released through a porous plug attached to the metallized plastic. In the event of a serious catastrophic gas buildup, the case cracks and spills its contents into the metallized plastic bag, and gas is still released through the plug.
The battery vent system according to the invention, is formed of the following components: 1) A battery case made of a rigid material such as an injection moldable resin having good permeability to gas, suitably ethyl cellulose or polypropylene in the case of hydrogen gas. The case has walls capable of rupture under fast buildup of gas pressure. 2) The battery case is tightly enveloped by an expandable, gas impermeable enclosure such as a metallized plastic film. The film envelope contains a vent permeable to the generated gas such as a plug made of porous resinous material. The metallized plastic film envelope expands in the envelope as pressure buildup outside the case and the gas exhausts through the plug. Once exhausted, the film envelope returns to its original shape.
The two-stage venting system increases shelf-life and operational life of the battery by safely releasing gas buildup in the battery case. The venting system is easily manufactured from readily available materials. The components of the system are mechanically simple, comprising a molded case, metallized film and a plug of porous plastic. The system is simple to fabricate involving steps of molding, folding and sealing. The venting system is found to be extremely reliable and cost effective.